As small computer systems decrease in cost, the cost of the associated keyboards becomes a larger percentage of the total manufacturing cost and becomes a prime target for cost reduction. At the same time, keyboards for these small computer systems must meet stringent operator entry performance requirements for alpha-numerics and must also enable the manufacturer thereof to provide many custom design features intended to provide a competitive advantage for the associated keyboards and systems. Consequently, a successful keyswitch or keyboard design must simultaneously meet the requirements of low-materials costs, good operator performance, durability, and low manufacturing costs for both standard and custom production runs. A keyswitch that meets these requirements would also be useful in small control panels or individual momentary closure switch modules.
The "flexible membrane" technologies which were developed for hand calculator keyboards provide favorable cost advantages. The designs of these keyboards however, have several unfavorable properties which restrict efficient operator performance at a computer terminal or increase materials and manufacturing costs. First, the switch makepoint during actuation is often located at the end of switch travel. The consequent lack of aftertravel shocks the operator's finger by preventing follow-through movements. Second, the designs very often have high activation forces and severely limited switch travel (less than 20/1000ths of an inch). Third, attempts to increase switch travel and to provide tactile feedback in the switch action by molding domes into the "flexible membrane" reduce switch durability because of "fatigue cracking" around the domes. In addition, since the snap action associated with the domes is still limited by a relatively short travel, the improvement in tactile feel is limited.
Most membrane switch designs incorporate a solid dielectric spacer sheet between a moving flexible sheet or membrane and an associated, nonmoving electrical contact facing it. This spacer sheet has apertures in it which correspond to each switch's electrical contacts. The flexible membrane is pressed through an aperture to contact the associated bottom circuit and make a "keyed" electrical connection. Another prior art switch utilizes a resilient material in place of the spacer sheet and also uses multiple graphic overlay sheets and a mesh screen used as the conductor; however, this construction produces a switch with high operating forces and limited actuation travel.
In summary, one of the above named membrane switch designs will permit high operator performance levels at a computer terminal or related utilization devices. This poor performance results from high operating forces, limited switch travel, and makepoints at the bottom of switch travel. Prior attempts to increase switch travel or to add tactile feedback to the switch designs discussed above either reduce the durability of the switches or increase their cost.